chore: use single-instance plugin as described by the docs

src-tauri/Cargo.lock

@@ -1018,6 +1018,7 @@ dependencies = [
  "tauri-plugin-macos-permissions",
  "tauri-plugin-opener",
  "tauri-plugin-shell",
+ "tauri-plugin-single-instance",
  "tempfile",
  "tokio",
  "tokio-test",
@@ -4500,6 +4501,22 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "tauri-plugin-single-instance"
+version = "2.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "97d0e07b40fb2eb13778e30778f5979347a2bf30e1b9d47f78ff7fe92d2e4b3d"
+dependencies = [
+ "serde",
+ "serde_json",
+ "tauri",
+ "tauri-plugin-deep-link",
+ "thiserror 2.0.12",
+ "tracing",
+ "windows-sys 0.59.0",
+ "zbus",
+]
+
 [[package]]
 name = "tauri-runtime"
 version = "2.6.0"